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The wide variety of corrosion control agents, biocides, and pH adjust chemicals do not allow a comprehensive list of chemical analyses and detection ranges. However, each combination of chemicals provides analytical challenges, which need to be effectively addressed in order to

Additional Monitoring Techniques

it includes the more frequently used methods. Any method should be matrix-tested to ensure that measurement results are not masked or biased by the presence of high concentrations of

corrosion inhibitors.

8.6.1 Ion Chromatography (IC)

This technique is routinely used to monitor concentrations of control agents such as nitrite, as well as contaminants like nitrate, chloride, fluoride, and sulfate (anions), or sodium, calcium, and magnesium (cations). The determination of nitrite will be mostly unaffected by all other species because nitrite is the major ionic species in the solution. However, because the concentration range of this ion can be so broad (500 to >1500 ppm), it will also act as an eluent in the

chromatography process. This can have several effects on the analytical process, most notably, variability of the detection limit and the retention time of the analyte.

The retention time shift should be determined by comparing the system chromatogram with chromatograms of standards used to calibrate the instrument, as long as the standards are run in the inhibitor matrix. The shift of retention time can also cause a broadening or sharpening of the chromatographic peak (dependent upon the eluent strength relative to the calibration). The detection limit is affected by the detector response to the inhibitor in the system, as well as the way the peak shape has deformed due to the presence of other ions in the system. Thus, if a conductivity detector is used, the detection limit will most likely increase at higher

concentrations of anionic materials (due to higher conductivity). However, the limit of detection of the contaminant analytes is not nearly as important as being able to reproduce values close to the Action Levels. Thus, it would be important to establish a quality check for each inhibitor system at the upper end of the inhibitor concentration, ensuring that Action Level values can be achieved.

For analysis of cations, the impact that borate or phosphate (added as part of the inhibitor package) can have on complexing with the metals should be evaluated under matrix match conditions.

8.6.2 Ion Selective Electrodes (ISE)

This method of analysis is based on the electrochemical potential change of a half-reaction based on the concentration of a specific ion when measured against an electrode, which is selective for the half-reaction of that ion. Because the method measures potential and not an intrinsic

parameter of the particular ion, it is subject to many different types of interferences, most notably other ions of similar charge and reduction potential. This method should also be verified

applicable in the concentration used for the CCW system, for the ions being determined. Major solution constituents, such as nitrite, can be effectively assessed because the contaminant concentrations are generally insufficient to alter the potential for the nitrite electrode. This has been used as an alternative to ion chromatography for chloride, fluoride, and sulfate analysis. There are ion selective electrodes for metal ions as well.

Ammonia analysis by electrodes can be performed using one of two different types. Ammonium ion is detected as described previously and the sample is made acidic with hydrochloric acid. This method suffers from interference from sodium ion. Thus, analysis of systems where the corrosion inhibitor is added as sodium salt will need to be assessed for the extent of interference. The second electrode method is by gas sensing electrode (GSE). In this technique, a membrane that is sensitive to the passage of ammonia gas is immersed in a sample that has had sufficient NaOH added to it to convert all of the ammonia to gas.

8.6.3 Atomic Absorption (AA) and Inductively Coupled Plasma (ICP)

Both of these methods are very useful at determining total metal ion concentration in solution. Analysis for iron, copper, magnesium, calcium, chromium, molybdenum, and sodium can all be routinely performed using these methods. Both methods measure an element-specific

characteristic based on atomic structure of the metal, making the analysis very specific for an individual element. For the types of matrices that are experienced in CCW systems, there are few, if any, matrix effects that would affect the analytical result. It is important to note that, in each of these analyses, the result is for total metal content. This analysis does not yield

information about the oxidation state of the material being examined, nor does it identify molecular species. This is best described by two different analyses.

Chromate is a good anodic corrosion inhibitor. It is also a strong oxidizing agent. If a CCW system containing chromates were to experience an intrusion of oil, it is likely that the oil would be partially oxidized to carbon dioxide and water, while the chromate would be reduced to Cr+3.

Depending on the concentration of Cr+3 and the pH, the Cr+3 might precipitate. The Cr+3 does not provide any anodic inhibition at all. Analysis of the water after the event would only indicate the total chromium, which would be a mixture of the two species. Such a result would lead to an erroneous conclusion about the appropriate level of corrosion inhibitor.

Molybdates are also good anodic inhibitors. However, this species can also be converted to MoO3, which has no corrosion inhibitor value at all. This species can form small particulates that

can become suspended. If total molybdenum analysis is performed, it again might indicate a satisfactory inhibitor level, when in fact it is low.

In each of these instances, a spectrophotometric test is available for the specific molecular species, which provides the inhibitor function. It might be advisable to use the alternate test on a certain frequency (for example, annually) to ensure that no degradation occurs. This will be verification that the AA or ICP methods are fairly reproducing the inhibitor concentration required.

8.6.4 Ultraviolet/Visible Spectroscopy (UV/Vis)

Additional Monitoring Techniques

Chloride, fluoride, and sulfate can also be determined, but this practice has become less common with the advent of ISEs and IC units. In general, a chromogenic agent, which has molecular absorbance at a specific wavelength, is used in the analysis. These molecular bands are very broad and interferences from other colored matter in the same wavelength range are

interferences. Sample turbidity can also provide interference in these analyses, so sample filtration might be necessary prior to analysis to ensure accurate measurements.

8.6.5 pH and Conductivity

These two methods of analysis measure the total effects of all the dissolved matter in the solution on the parameter of hydrogen ion concentration (pH) and solution electrical conductivity. These tests are used as measures of gross contamination or loss of corrosion control agent. Together, these can be used on a more frequent basis than the element- or inhibitor-specific tests to ensure that system chemical composition remains relatively the same. This parameter is useful because the intrusion of an unknown contaminant will most likely affect these two tests, which would be a warning to begin an investigation.

One particular intrusion, which would not necessarily show up as affecting these two analyses, would be from solvents or oils. In these cases, odor, color, or TOC would be helpful in

diagnosing the source of the contaminant.

Care must be taken when performing pH on inhibited glycol coolants. In the case of coolants greater than 50% volume glycol, pH values are not actual values according to the accepted definition, but are apparent pH values, which are useful in interpretation of coolant condition [37].

8.6.6 Vendor Test Kits

Most vendor test kits are made for field use as a first-order measure of the concentration of a particular analyte. The methods are almost always based on sound analytical methods that have been modified for a portable test kit. In almost all cases, the analyst is asked to interpolate scale readings (using a mini-meter) or judge a color against a standard set of colors (using a color comparator). For systems that are relatively stable, with no contaminant ingress and little change in process, these might be acceptable for routine analyses. However, if these are used for CCW systems, other laboratory methods should be used on a routine basis to ensure that trends are not developing that might prove harmful to the system.

Vendor test kits can suffer from a myriad of interferences, the most notable are colored contaminants. A specific example occurs with the analysis of glutaraldehyde (a di-aldehyde). The reagent used is sensitive to the glutaraldehyde molecule yielding a certain color. Partial decomposition products of glutaraldehyde will also yield a color with this material, but not of exactly the same wavelength. The analyst must then discern, visually, whether or not the color of the material being tested is the same as that of the standard.

8.6.7 X-Ray Fluorescence (XRF) of Filtered Material

XRF is a routinely used method of analysis for metals. It is very effective for analysis of insoluble materials filtered from a system. The instrument produces a highly focused electron beam on the solid material on the filter surface and this results in the production of characteristic x-rays of the elements. Most commercial instruments have the capability of analyzing in the range of 3–20 keV. This is the region for the Kα x-ray lines for all of the first-row transition

elements and molybdenum. The Kα x-ray lines are the most sensitive for any element. This

region can also be used for Lα lines of some of the higher atomic number elements, such as lead.

This can potentially be used to differentiate between insoluble and dissolved metals. It is very important to ensure that the sample point and process provide a representative flow for the sampling of particulates in the fluid medium.

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